Obesity is the most common nutritional problem in developed nations. Although cause and effect are not clearly established, obesity has a high association with coronary artery disease, hypertension, maturity onset diabetes, and excess mortality. Medical treatment of obesity has been generally unsatisfactory. However, the recent development of three new techniques -behavior modification, semistarvation, ketogenic regimens, and surgical therapy - has ushered in a new era where effective therapy can now be offered to many patients. The protein sparing modified fast (PSMF) in adults offers significant advantages as a prototype hypocaloric regimen including absence of hunger, rapid weight loss, and sparing of lean body mass. It is the purpose of this study to investigate the mechanism and degree of protein sparing (hormonal-substrate profiles, nitrogen balance, whole body protein flux, metabolic rates) in adults while consuming a PSMF /1.5g protein/Kg ideal body weight (IBW)/or a variant diet accomplished by the isocaloric substitution of some protein by glucose (.8g protein/Kg IBW, .7 g glucose/Kg IBW). The effect of these two diets and of exercise on these metabolic parameters, rate of weight loss, and outpatient dietary adherence will be explored in the design of optimal diet composition for weight loss. A preliminary approach to the development of a comprehensive treatment program of obesity including a PSMF, behavior modification, nutrition education, and exercise will be undertaken.